1. Field of the Invention
The present invention relates to a fan having edge portions of blades connected by a band, and a shroud assembly adopting the fan.
2. Description of the Related Art
As shown in FIG. 1, a conventional fan 10 includes a hub 11 coupled to a driving shaft of a motor or engine and a plurality of blades 12 formed on the outer circumferential surface of the hub 11 spaced apart from each other at equal intervals. A shroud 20 includes a housing portion 21 for housing the fan 10 and a plurality of stators 23 supported by the housing portion 21 and connecting to a motor support portion 22.
In the operation of the above fan-shroud assembly, as the fan 10 rotates, airflow is generated since the blades 12 are disposed at an angle with respect to the driving shaft. Here, the airflow generated by the blades 12 has two directional components, that is, an axial component and a radial component. The radial component gradually increases from the center of the fan 10 toward the free end of each blade 12. Accordingly, at around the edge of the fan 10, airflow directs outward along the radius direction and thus collides against the inner surface of the housing portion 21 of the shroud 20, which acts as a resistance force reducing the airflow efficiency of the fan 10. Such a resistance force not only lowers the efficiency of the fan 10 but also causes noise.
Also, the noise is usually generated around the free end of the respective blades 12 while the fan 10 rotates. That is, when the fan 10 rotates at high speed, air around the free end of each blade 12 flows from a positive pressure surface toward a negative pressure surface, generating a vortex. The vortex increases with increments of a centrifugal force of the airflow as the fan 10 rotates more rapidly. The vortex interfering with the housing portion 21 of the shroud 20 encompassing the fan 10 generates interference noise increasing the blade passing frequency (BPF: the number of rotations of the fan.times.the number of blades/60).
To solve the above problems, a solution to reduce the noise by forming a portion of a leading edge of the free end of the blade to protrude toward the upper stream in an axial direction with respect to the housing has been introduced. However, the interference noise generated by the housing of the shroud remains at a trailing edge portion of the free end of the blade.
Also, to reduce such noise, a band 13 connecting each free end of the blades 12 has been provided as shown in FIG. 2. In such a case, the width of the free end of the blade 12, i.e., the chord length, cannot be increased due to limitations in the width of the band 13. Therefore, reduction of noise cannot be achieved by increasing the chord length.